Such dual-channel systems are known for mixing a cold and a warm air stream, enabling quick and reliable temperature control. This is an interesting possibility in order to lower the costs for installation and energy in a ventilated building since it eliminates the need for pipe systems for hot and cold water outside the machine room. Several makes were on the market during the 70s and 80s and many systems were installed.
However recent development has departed from this technology in favour of water-borne heating and cooling, such as fan coil units or cooling ceiling combined with radiators. One reason was that the known mixing boxes entailed certain drawbacks, the dampers in the mixing boxes having a tendency to clog after some time in use, for instance, and that overflow often resulted between the supply channels so that the fundamental principle of the system entailing the supply of air at different temperatures was disrupted. Another problem with the known installations was that their mixing boxes require a considerable amount of space and must be installed in each room to be ventilated. The boxes also require pipes to be laid that take up relatively much space.